The present invention relates to the dyeing of fibrous articles containing polyamide fibers with anionic dyes.
Anionic dyes such as acid dyes and pre-metallized dyes are widely used for the dyeing of polyamide fibers in which the nitrogen-containing groups of the polyamide polymer serve as dye sites. In conventional dyeing processes using such dyes, articles containing the polyamide fibers are immersed in an aqueous bath containing a solution of the dye after any pre-treatment processes such as scouring. While a wide variety of dyeing equipment is used, it is typical for all of the dye to be used in the process to be present in the bath initially. The bath containing the dye and the article to be dyed is also usually at a very low initial temperature, e.g., 80.degree.-120.degree. F. (26.7.degree.-48.9.degree. C.) increased gradually to an elevated temperature often as high as the boiling point as the dyeing progresses.
While high quality dyeing can be achieved using the conventional dyeing process for some acid dyes such as small molecule "levelling" dyes, dye cycles to achieve levelling with such anionic dyes are sometimes extremely long and are therefore costly. Moreover, with large molecule acid and pre-metallized dyes which are desirable for applications requiring good light and/or wash fastness, there are often severe dye uniformity problems associated with the conventional dyeing process.
Large molecule acid and pre-metallized dyes are often referred to as "structure sensitive" dyes since non-uniform dyeing can result from even minor, and otherwise undetected, variations in the fiber physical structure. While dye-levelling and/or retarding agents can be added to the dye bath to improve dyeing uniformity, such agents sometimes provide only limited increases in dye uniformity and usually have disadvantages including increased initial expense and higher cost to treat the spent dyeing bath. In addition, because of their retarding effect, such chemical agents can sometimes increase dyeing cycles or make it difficult to obtain deep colors or dark shades. Also, dye yields from anionic dyes, i.e., the strength of color produced from a given quantity of dye on the fiber, are sometimes not as high as desired.